Most craft stores have the Wilton ready to use buttercream, but in my opinion it doesn't taste very good.

do you have any cake supply stores in your area? Or check with your local baskin robbins to see if you can buy a bucket from them. Or some of the other grocery stores. Just talk to the bakery managers.

Idon't know if they will, but they might. Each baskin robbins is a franchise so each manager will decide if they are willing. But it doesn't hurt to ask and I know they use buttercream for the decoration parts.